The present invention relates to a light-sensitive composition comprising a novel diazo resin which has an improved developability when it is developed with an alkali developer after imagewise exposure to light and which has also an improved adhesion to a substrate having a hydrophilic surface, as well as a presensitized plate for use in making a lithographic printing plate (hereunder referred to as "PS plate") wherein the diazo resin is used as a constituent of a light-sensitive layer or it is used as an underlayer and which can provide a lithographic printing plate free of background contamination.
Up to now, there have been used a variety of light-sensitive materials or compositions for preparing light-sensitive layers of PS plates. Most of the light-sensitive materials used for manufacturing negative-working PS plates are diazonium compounds, in particular polymeric diazonium salts (so-called diazo resins) in which at least two diazonium salt skeletons are linked.
When such a diazo resin is applied onto the surface of a proper substrate such as paper, a plastic plate or a metal plate and then exposed to actinic rays through a negative transparency, the diazo resin present on the exposed portions is decomposed and thus made insoluble in water and lipophilic. On the other hand, the surface of the substrate is exposed by dissolving out the unexposed portions with water. Therefore, if a substrate whose surface has previously been made hydrophilic is used, the unexposed resin portions are removed by development and hence the hydrophilic surface of the substrate is exposed. Thus, such portions receive water and repel ink during printing on press operation. On the other hand, the diazo resin present on the photolytically decomposed portions (exposed portions) exhibits lipophilic properties, thus repel water and receive ink. Consequently, these kind of light-sensitive materials provide so-called negative working presensitized lithographic printing plates.
The linking of the diazonium salt skeletons has conventionally been performed by condensing an aromatic diazonium compound with a reactive carbonyl group-containing organic condensation agent, in particular an aldehyde such as formaldehyde or acetaldehyde or an acetal in an acidic solvent. Most typical examples thereof are p-diazodiphenylamineformaldehyde condensates. The method for preparing the diazo resins in which the foregoing linking method is adopted are detailed in, for instance, U.S. Pat. Nos. 2,679,498; 3,050,502; 3,311,605 and 3,277,074.
If the counter anion of the diazonium salt in the diazo resins is an inorganic anion of mineral acid such as hydrochloric acid, hydrobromic acid, sulfuric acid or phosphoric acid or its complex salt with zinc chloride, the resulting diazo resins are water-soluble and thus unstable to moisture, which deteriorates their storage stability. For this reason, Japanese Patent Publication for Opposition Purpose (hereunder referred to as "J. P. KOKOKU") No. Sho 47-1167 and U.S. Pat. No. 3,300,309 propose methods for making a water-soluble diazo resin water-insoluble by reacting the diazo resin with an organic coupling agent such as a phenolic hydroxyl group-containing aromatic compound, an acidic aromatic compound or an acidic aliphatic compound, to improve its storage stability.
However, the diazo resins of this kind have insufficient solubility in organic coating solvents such as alcohols, ketones and glycol ethers. To solve this problem, i.e., to make the diazo resins of this kind water-insoluble and organic solvent-soluble, Japanese Patent Unexamined Publication (hereunder referred to as "J. P. KOKAI") Nos. Sho 54-98613 and Sho 56-121031 propose the use of diazo resins whose counter anion is selected from those derived from halogenated Lewis acids such as tetrafluoroboric acid and hexafluorophosphoric acid and perhalogenic acids such as perchloric acid and periodic acid.
Moreover, J. P. KOKAI Nos. 58-209733, Sho 62-175731 and Sho 63-262643 propose the use of an anionic surfactant, more specifically sulfonic acids having a long chain alkyl group as a counter anion for diazo resins.
However, these diazo resins are essentially alkaline water-insoluble in nature, the diazo resins are not completely removed during development with an alkaline developer and thus a part of the unexposed portions of the light-sensitive layer remains unremoved. This results in the formation of lithographic printing plates having background contamination.
On the other hand, many attempts have been made to use photopolymerizable compositions as materials for light-sensitive image-forming layers of PS plates. Such light-sensitive compositions include basic ones comprising a polymer as a binder, a monomer and a photopolymerization initiator as disclosed in U.S. Pat. No. 3,458,311; compositions whose hardening efficiency is improved by introducing unsaturated double bonds into a polymer serving as a binder such as those disclosed in J. P. KOKOKU No. Sho 49-34041; and compositions in which novel photopolymerization initiators are employed such as those disclosed in J. P. KOKOKU Nos. Sho 48-38403 and Sho 53-27605 and British Patent No. 1,388,492.
Photocrosslinkable materials which crosslink through a cyclization-addition reaction have also been well-known and have been widely employed as a principal component for light-sensitive compositions for use in making light-sensitive layer of PS plates. Useful photocrosslinkable polymers include those having a maleimido group at a side chain; and polymers having a cinnamyl, cinnamoyl, cinnamylidene, cinnamylindenacetyl and/or calcon group, which groups have a photodimerizable unsaturated double bond adjacent to the aromatic nucleus, at a side chain or in the main chain. Some of them have already been practically used. In particular, those having a relatively high sensitivity are both polymers having a maleimido group at a side chain and polyester resins which are prepared by condensing phenylene diacrylic acid or its alkyl esters with a glycol and which have a cinnamic acid skeleton in each molecular chain. From the viewpoint of safety in working environment, developers for the light-sensitive compositions containing these photocrosslinkable compounds are preferably free of organic solvents. Therefore, it has been tried to make these polymers alkaline-soluble. Such alkaline-soluble polymers are copolymers of N-[2-(methacryloyloxy)ethyl]-2,3-dimethylmaleimide with methacrylic acid or acrylic acid, as disclosed in Die Angewandte Makromolekulare Chemie, 1984, 128; and polymers having a photodimerizable functional group and a carboxyl group at a side chain, as disclosed in J. P. KOKAI Nos. Sho 62-175729, Sho 62-175730, Sho 63-25443, Sho 63-218944 and Sho 63-218945.
However, when the foregoing photopolymerizable or photocrosslinkable polymers are used for the light-sensitive layer of PS plates, the adhesion between the layer and an aluminum substrate is insufficient. As a result, the images are easily damaged and/or peeled off by, for instance, rubbing with a brush during the development (in other words, the image portions do not have sufficient strength) and are also possibly peeled off during printing (insufficient printing durability). In particular, this tendency becomes conspicuous under low exposure condition and in such a case, the sensitivity of the light-sensitive layer is reduced.
To eliminate such problems of insufficient adhesion between the light-sensitive layer and the substrate, many attempts have been made. There are proposed a variety of means for improving the adhesion, for instance, the use of a negative-working diazo resin as a material for an underlayer (see J. P. KOKAI No. Sho 54-72104 and U.S. Pat. No. 3,905,815); the addition of a diazo resin to a light-sensitive composition (J. P. KOKAI No. Sho 61-38943); the use of a negative-working diazo resin in the light-sensitive layer or as an underlayer (see J. P. KOKOKU No. Sho 50-7481 and J. P. KOKAI No. Sho 62-78544); and an attempt of using various adhesive layers, for instance, the use of underlayers of polyester resins, polyurethanes and epoxy resins. On the other hand, there have also been proposed various method for improving the adhesion of a substrate to a light-sensitive layer. For instance, J. P. KOKOKU No. Sho 46-26521 proposes the use of a substrate obtained by anodizing an oxidized layer provided on an aluminum substrate in a phosphoric acid solution; J. P. KOKAI Nos. Sho 49-8428, Sho 49-12903, Sho 49-93101 and Sho 50-138903 propose aluminum substrates which are electrolyzed in a sulfuric acid solution and then etched with phosphoric acid, polyphosphoric acid or an alkaline solution, to enlarge the pore size of an anodized layer.
However, according to the above methods in which the adhesion of the anodized layer on a substrate to a light-sensitive layer is improved, the adhesion is excessively enhanced due to anchoring effect. In addition, the non-image areas are liable to receive a printing ink during printing and to cause background contamination. Moreover, when a PS plate, for instance, one having a light-sensitive layer containing a negative-working diazo resin is developed with a developer consisting of an alkaline water, the solubility of the light-sensitive layer in an alkaline developer is insufficient and, as a result, a part of the diazo resin and the light-sensitive layer on the non-exposed portions remain unremoved. This leads to background contamination during printing.
Moreover, the light-sensitive layer of the PS plate must be easily removed from the substrate during the development while it must be firmly adhered to the substrate after the development (in the image areas) and during printing operations. However, the methods wherein a variety of adhesive layers are used cannot simultaneously satisfy the foregoing two requirements which are contrary to one another.